1. Field of the Invention
The invention relates to a lithium secondary battery. More specifically, the invention relates to a lithium secondary battery that includes a positive electrode having an amorphous positive electrode active material, a negative electrode and a nonaqueous electrolyte. In addition, the invention relates to a use of the lithium secondary battery.
2. Description of the Related Art
A lithium secondary battery (typically, a lithium ion battery) is charged or discharged in such a manner that lithium ions migrate between a positive electrode and a negative electrode. The lithium secondary battery has a light weight and a high power, so a demand for the lithium secondary battery is expected to increasingly grow as a vehicle-mounted power supply, a power supply for a personal computer and a power supply for a mobile terminal in the future. Japanese Patent Application Publication No. 2008-251481 (JP-A-2008-251481) describes a lithium secondary battery.
One typical configuration of a lithium secondary battery of this type includes an electrode in which a material (electrode active material) that is able to reversibly occlude and release lithium ions is held by a conductive member (electrode current collector). A typical example of an electrode active material (positive electrode active material) used for a positive electrode is a lithium transition metal composite oxide, such as a lithium-nickel-based oxide and a lithium-cobalt-based oxide. Another example of the positive electrode active material is a so-called olivine-type phosphate compound containing lithium (for example, LiMnPO4, LiNIPO4, and the like). The olivine-type phosphate compound has a high theoretical capacity, reduces cost and has a high degree of safety, so the olivine-type phosphate compound receives attention as a promising positive electrode active material. The olivine-type phosphate compound of this type is described in Japanese Patent Application Publication No. 11-025983 (JP-A-11-025983).
Incidentally, a battery is generally used (charged or discharged) within the range of potential that takes into consideration the decomposition potential of a nonaqueous electrolyte. However, the olivine-type phosphate compound, such as LiMnPO4, has a redox potential higher than that of a lithium transition metal composite oxide, such as a lithium-nickel-based oxide and a lithium-cobalt-based oxide. Therefore, in a battery configuration that uses a general electrolyte in which a carbonate-based solvent, or the like, is used as a nonaqueous solvent, decomposition of the electrolyte occurs if the potential of the positive electrode is excessively increased, so there is a problem that the intrinsic capacity of the olivine-type phosphate compound cannot be sufficiently utilized (when trying to use a battery within a range in which the potential of the positive electrode does not excessively fall above the decomposition potential of the electrolyte, a high capacity cannot be obtained) or the flexibility of selection of the composition of the electrolyte is low (the number of choices of a nonaqueous solvent is small, that is, the nonaqueous solvent is limited to the one having a relatively high decomposition potential).